Doc: improve footnote documentation (2547) (issue 6498109)

[tdanielsmusic]

Reviewers: ,

Message:
I've already spotted several nit-picking things I'd
like to change, now I get a slightly different view
on Rietveld, but I'll leave those as an exercise for
the reviewers ;)  In any case I expect there will be
more substantial comments to deal with first.

Trevor


Description:
Doc: improve footnote documentation (2547)

 - clearer headings, separating footnotes in music
   expressions from those in stand-alone text

 - distinguish Event- and Time-based footnotes (thanks
   David)

 - explain how offsets are measured from grob boundary

 - use examples that cover just single points for clarity

 - explain how to create footnotes in stand-alone text
   with automatic and with manual marks

   This continues the work begun in 6261048 by David

Please review this at http://codereview.appspot.com/6498109/

Affected files:
  M Documentation/notation/input.itely